The present invention relates generally to eddy current inspection and, more particularly, to eddy current inspection of aircraft structures near fasteners.
Aircraft structures are generally constructed using multiple layers of material utilizing lap joints and fasteners, such as rivets. Sealants are often disposed between the layers to prevent corrosive materials from collecting in the joint. In response to stress exerted on the aircraft structure during operation, cracks occasionally develop in the layers comprising the aircraft structure in the vicinity of the fasteners.
In order to ensure the structural integrity of the aircraft, aircraft structures are repeatedly inspected during the life of the aircraft, to determine that cracks have not formed near the rivet hole. However, these inspections can be difficult to perform, due to the reduced access to critical inspection areas, such as portions of the aircraft structure surrounding the rivets. The inspections are further complicated by the fact that cracks internal to lap joints can be very difficult to detect.
Currently, aircraft structures are inspected for cracks internal to lap joints using x-ray, eddy current, and ultrasonic inspection techniques applied to the external surfaces of the lap joints. However, application of these inspection techniques is often impeded by the internal structure of the aircraft and frequently requires considerable aircraft disassembly. In addition, ultrasonic signals can be difficult to interpret because of the complicated geometry near the rivet and the number of interfaces in the lap joint. Eddy current inspection may undesirably require removal of the rivet and disassembly of the structure and is further complicated by the presence of sealants in the joints. X-ray inspection creates radiation exposure problems and requires evacuation of operators during testing, preventing concurrent maintenance and inspections.
By way of further background, eddy current inspection is based on the principle of electromagnetic induction in which a drive coil is employed to induce eddy currents within the material under inspection, and secondary magnetic fields resulting from the eddy currents are detected by a sense coil, generating signals which are subsequently processed. Eddy current inspection detects flaws as follows. The presence of a discontinuity or a crack in the surface of the component under inspection changes the flow of the eddy currents within the test specimen. The altered eddy current, in turn, produces a modified secondary magnetic field, which is detected by the sense coil, thereby generating a signal which indicates the presence of the flaw upon subsequent processing.
Previous eddy current inspection applications to aircraft structure involved positioning a single eddy current coil probe adjacent to the surface of the aircraft structure. Although this technique is adequate for external and easily accessible surfaces, it is not desirable for interior surfaces. Due to gaps between aircraft structure layers filled with air or sealants, low frequency eddy current inspection must be used to inspect interior surfaces from an external position. However, low frequency eddy current inspection provides limited resolution. Thus, in order to perform high resolution eddy current inspection of interior surfaces of a lap joint using this technique, the rivets would have to be removed and the structure disassembled. Accordingly, inspection times would be considerable to disassemble, inspect, and reassemble the aircraft structure. Further, the cost of labor required to perform these tasks would be high.
Moreover, the conventional eddy current inspection technique is performed during maintenance periods when the aircraft is taken out of use, disassembled, inspected, and reassembled. In particular, this conventional technique does not inspect for crack formation during flight operations.
Accordingly, it would be desirable to provide a method and apparatus for performing eddy current inspection on interior surfaces of multilayer structures that does not require disassembly of the multilayer structure. In addition, it would be desirable to provide a method and an apparatus for performing eddy current inspection on a lap joint near a fastener, that does not require disassembly of the fastener or the lap joint. It would further be desirable for the method and apparatus to permit periodic eddy current inspection of the lap joints and other multilayer structures, that provides information about the presence and size of cracks, such as cracks near fasteners (e.g., rivets) in the multilayer structure. In addition it would be desirable to provide a method and apparatus for performing eddy current inspection during flight operations.
Briefly, in accordance with one embodiment of the present invention, an embedded eddy current inspection apparatus includes a substrate having an opening, and a test eddy current coil affixed to the substrate near the opening.
In accordance with another embodiment, an internally inspected multilayer component structure includes an upper layer and a lower layer. The multilayer component structure further includes an eddy current probe embedded between the upper and lower layers. The eddy current probe includes a test eddy current coil facing a subject layer selected from the upper and lower layers.
In accordance with another embodiment, a method of inspecting a multilayer component structure including an upper and a lower layer is provided. The inspection method includes energizing a test eddy current coil embedded between the upper and lower layers. The test eddy current coil faces a subject layer selected from the upper and lower layers. The inspection method further includes energizing a reference eddy current coil simultaneously with the test coil. The reference eddy current coil is embedded between the upper and the lower layers in a reference region of the multilayer structure and faces the subject layer. The inspection method further includes comparing a test signal from the test eddy current coil with a reference signal from the reference eddy current coil, to determine whether a flaw is present in the subject layer near the test eddy current coil.
In accordance with another embodiment of the invention, an embedded eddy current inspection system is provided for inspecting a multilayer component structure including an upper and a lower layer. The inspection system includes an eddy current probe embedded between the upper and lower layers. The eddy current probe includes at least one test eddy current coil. The system further includes a signal generator configured to energize the test eddy current coil. The system also includes a comparison module for comparing a test signal received from the test coil and a reference signal and outputting a compared signal.